It's not the healthiest thing for the potential sneezer because stopping a sneeze can cause a backup of pressure to the sinuses, or maybe to ears through Eustachian tube. However, it's healthier to anyone around the potential sneezer, and it's far less disgusting than sneezing; so sometimes a person thinks risking a little pressure back up may be worth it. (I've been stopping sneezes for decades. So far, so good. Knock on wood.) :)
As far as the argument about sneezing meaning there's something that should be gotten rid of anyway goes... That's not necessarily always true, at least not as people often think of "need to get rid of it" goes. For example, every time I have an Altoids I first have what I call "my Altoid sneeze" (which do stop). Someone else I know sneezes when first taking an Altoid too. While the "business" in the back of the throat may be "rejecting" that first blast of mint temporarily, the sneeze comes from something other some "horrible, dirty" thing that the body needs to be rid of. It's just a reaction to the mint. (Sometimes sneezing just happen for "goofy" reasons - not because there's something all that horrible that caused the sneeze.)
In any case, my thinking is that while there's no question that stopping a sneeze could bring some problems, it's worth the risk. (lol) If it's something other than an Altoids or some other little "quirky" thing causing the urge to sneeze, than people should blow their noise, take an allergy pill, or "whatever". There are other ways to address the issue of getting rid of bad stuff. So, while it may not be the healthiest thing - I plan to keep catching my sneezes at least until I blow out my ear drums (at which time I'll come back here and point out why I shouldn't have made the decision I did about one particular sneeze). :)
In all seriousness, one person's "harmless germs" (that don't cause him any real problems) can be deadly to another person (especially babies, elderly people, and anyone with a weakened immune system). Better is risk a pressure back-up than possibly risk someone else's health in a far more serious way.